Coulometric D2 flow control?

[Rich Feldman]

In the last year or two we have seen some fusor success stories using D2 from electrolysis of D2O in plastic PEM fuel cell demonstration kits.

How about skipping the inverted-test-tube gas reservoir, and setting the flow rate by regulating the electrolysis current? If my figures are right, 143 milliamperes would evolve 1 sccm of hydrogen and 0.5 sccm of oxygen. *

There are a couple of issues that might make this nontrivial.

1. Can any readers here tell us the electric current rating of their PEM cells?

2. How 'bout their mechanical strength? Would cell crush or membrane burst if H2 port were connected to vacuum?
We might operate the whole cell in a sealed box, connected to a tee on vacuum system's roughing line. Then how would the cell be cooled?
Or we could put a traditional gas flow restrictor between hydrogen port and vacuum chamber. Absolute pressure would depend on the electrolysis current and the flow restrictor characteristics.

[edit] * For review, 1 scc is 4.46e-5 mol of gas, if ideal. That much H2 or D2 requires 8.92e-5 mol of elementary charge = 8.61 coulombs. For comparison, those glass-capillary hourmeters move 1 cubic mm of mercury for each 13 coulombs. [\edit]

[Bob Reite]

That's how I operate mine! To keep the PEM cell from being damaged, I have a pressure limiting system comprised of two vials filled slightly less than half full with vacuum oil connected with tubing. A complete description of my gas system is archived at: viewtopic.php?f=18&t=9599#p64842 The worst that can happen is either you start venting D2 to atmosphere if your PEM cell output exceeds demand once the collection vial is full, or you start sucking in air if the demand exceeds supply and the collection vial is emptied.

If I have a flow rate of 5 sccm, setting the electrolyzer current to around 800 mA is about right to keep up. This is more than the calculated value, but that calculation I think is based on perfect cell. The real world cell has losses, it will get slightly warm to the touch after a long run. The cell I had stated that it will draw 2.5 A at 3 volts, so I designed the power supply to be adjustable from 1.5 V to 3 V. I just monitor the current. That voltage range gives nearly zero output to slightly over 2.5 A, although the cell current will drop as the D2O gets consumed, until it is replenished.

In all my experiments, including the false starts, I've only used 35 grams or so of D2O

[Peter Schmelcher]

I was originally going to control the PEM gas generation rate by cell current but opted instead for a Mass Flow Controller. At the time when I got my MFCs they were less expensive than metering valves and orifices, they start at $20. Using a MFC a batch process with bottle storage seems like the most reasonable structure.

I read someplace that the membrane burst pressure is 15psi. Personally I would keep the pressure differential small and slightly higher on the D2 side.

The Hydrocar toy PEMs are ok at 4.5A and produce 36sccm of D2 but they get overly hot so cooling is needed.

Two short videos of a PEM fuel cell operating at 4.5A and 3A.
http://youtu.be/huiVgMdcNXQ
http://youtu.be/9DKmkQrZ9Rs

-Peter

[Rich Feldman]

Anybody know if PEM electrolysis cells have a shelf life, in their new (never wet) state?
For long term storage after use, should they be kept wet? Is there a practical way to get them dry inside?

Maybe the answers are in product manuals. I just received a Hydrocar kit via ebay, but have not yet opened its shrink-wrap film. The product box came inside a much larger shipping box, with nothing but air between them. I would have expected better packing. Vendor may have been unhappy to get only her $25 opening bid.

[Bob Reite]

Exposure to atmospheric impurities degrades PEM cells. The manual that came with mine recommends keeping the PEM cell stored in a zip lock bag when not in use. It also advises the use of distilled water for maximum life, the deuterium oxide you will be using should be pure enough. I would think that storing it "wet" would be better as that would block anything else from touching that side of the membrane. When my system is going to be idle, I leave the oxygen side full of deuterium oxide and plug all the ports.

[Rich Feldman]

Thanks for the input, Bob.
My online shopping for D2O turned up some heavy water trucks!
Only a few are as big as this one, with 41,700 lbs net capacity.